Energy Cells such as rechargeable energy storage devices, e.g. ultracapacitors, supercapacitors, Licaps, batteries, fuel cells are usually placed in series to reach a higher DC voltage.
A cell can be an individual Cell (1 Ultracap, 1 Licaps, or 1 battery cell, 1 fuel cell, etc) or can also be a series of more cells (e.g. 4 liCaps, 4 Ultracaps, . . . or 6 or 8 . . . ).
In battery management systems the voltage can be measured of the individual cells or from a few cells in series. For measuring the voltage there is preferably a trustworthy connection between the electronics and the cells.
A rechargeable energy storage device can comprise a power source, a string of cell modules in series, and a set of balancing/monitoring circuits, one per module as shown in FIG. 1. Each cell module can comprise an individual cell or a series of cells. A switch is controlled in the primary of a transformer and decides when a module has to be balanced. The cell voltage is measured, for example through an AD converter.
The numbering of the connections X.Y to a cell module in the figures is as follows:                1.X is the connection to the minus electrode of module X        2.X. is the connection to the plus electrode of module X        3.X− is the connection of the negative electrode of module X to the balancing circuit        3.X+ is the connection of the positive electrode of module X to the balancing circuit.        
The cell modules 12.1, 12.2, 12.3, 12.4 each contain a monitoring and control circuit 15.1 . . . 15.4 respectively, a switch SW.1, . . . SW.4, filter F.1 . . . F.4 such as an inductor and a filter circuit FC.1 . . . FC.4. The monitoring and control circuit (15.1 . . . 15.4) for example implemented as a microcontroller, is adapted to measure the cell module voltage of the cell associated with the module. Each module usually has a filter circuit (FC.1 . . . FC.4) connected to the analog input P+, and filter F.1 . . . F.4 connected to the analog input P− of the monitoring and control circuit (15.1 . . . 15.4).
The numbers 11, 21, 31 and N1 refer to galvanic isolation between a power source and an input to each module 12 and its switch SW.
The voltage on the input V after the galvanic isolation 11, 21, 31 and N1 must be higher than the cell voltages. Switch SW (SW1 . . . SW4) is a switch that can be used for active balancing systems. When switch SW is closed the cell module is charged to voltage V.
U.S. Pat. No. 5,254,930 A discloses a Fault detector for a plurality of batteries in battery backup systems. Here are 2 switches used at the primary of a transformer (not on the side of the cells) in the charge circuit. The voltage and current to the cells are measured and if a fault condition is measured (to high charge current to the cell, . . . ) the switches from the charger are going in shutdown.
U.S. Pat. No. 5,666,040 A discloses a Networked battery monitor and control system and charging method. The voltage is measured and at overvoltage a bypass resistor is switched with a switch to discharge the cell. This switch can't be used for a connection check.
U.S. Pat. No. 6,844,703 B2 discloses a Battery cell balancing system wherein preferential charging occurs for a cell with the lowest state of charge. At least one current limiting device is electrically coupled to transformer/rectifier circuits and a power supply. The current limiting device buffers a source voltage from a reflected voltage of at least one of the plurality of cells. A method of performing the same is also provided.